The invention relates to the field of electrochemistry and describes a catalyst-coated membrane and a membrane-electrode assembly produced therefrom for electro-chemical devices, for example fuel cells, electrolyzers or electrochemical sensors. Furthermore, a process for producing the catalyst-coated membrane and membrane-electrode assemblies is disclosed and their use is described.
Fuel cells convert a fuel and an oxidant at separate locations at two electrodes into electric power, heat and water. The fuel employed can be hydrogen or a hydrogen-rich gas, and the oxidant used can be oxygen or air. The energy conversion process in the fuel cell has a particularly high efficiency. For this reason, the fuel cells are becoming increasingly important for mobile, stationary and portable applications.
Owing to their compact construction, their power density and their high efficiency, membrane fuel cells (PEMFC, DMFC, etc.) are particularly suitable for use in a wide variety of fields.
For the purposes of the present invention, a fuel cell stack is a stack of fuel cell units. A fuel cell unit will hereinafter also be referred to as a fuel cell for short. It comprises a membrane-electrode assembly (“MEA”) which is located between bipolar plates, which are also referred to as separator plates and serve to bring gas to the unit and conduct away electric current.
A membrane-electrode assembly comprises an ion-conducting membrane which is provided on both sides with catalyst-coated reaction layers, viz. the electrodes. One of the reaction layers is configured as anode for the oxidation of hydrogen and the second reaction layer is configured as cathode for the reduction of oxygen. Gas diffusion layers (abbreviated as “GDLs”) made of carbon fiber felt; carbon fiber paper or woven carbon fiber fabrics are applied to these catalyst layers. The GDLs provide good access for the reaction gases to the electrodes and readily conduct away the cell current. For the purposes of the present invention, such an arrangement will be referred to as a five-layer membrane-electrode assembly (“5-layer MEA”). In contrast to this, there is the ion-conducting membrane coated with catalyst on the front and reverse sides which is referred to as 3-layer CCM (“catalyst-coated membrane”). It contains no gas diffusion layers. If only one side of the ion-conducting membrane is coated with catalyst, this is referred to as a two-layer catalyst-coated membrane (“2-layer CCM”).
Anode and cathode generally contain electrocatalysts which catalyze the respective reaction (oxidation of hydrogen or reduction of oxygen). As catalytically active components, preference is given to using the metals of the platinum group of the Periodic Table of the Elements. The majority of catalysts used are supported catalysts in which the catalytically active platinum group metals have been applied in finely divided form to the surface of a conductive support material. The mean crystallite size of the platinum group metals is from about 1 to 10 nm. Finely divided, conductive carbon blacks save been found to be useful as support materials.
The ion-conducting membrane preferably comprises proton-conducting polymer materials. These materials will hereinafter also be referred to as ionomers for short. Preference is given to using a tetrafluoro-ethylen-fluorovinyl ether copolymer bearing sulfonic acid groups. This material is, for example, marketed under the trade name Nafion® by DuPont. However, it is also possible to use other, in particular fluorine-free ionomer materials such as doped sulfonated polyether ketones or doped sulfonated or sulfinated aryl ketones and also doped polybenzimidazoles. Suitable ion-conducting membranes are described by O. Savadogo in “Journal of New Materials for Electrochemical Systems” I, 47-66 (1998). For use in fuel cells, these membranes generally need to have a thickness of from 10 to 200 microns.
The present invention describes catalyst-coated membranes (CCMs) and membrane-electrode assemblies (MEAs) having an integrated sealing material. The products according to the invention have a simplified, material-conserving structure and can therefore be produced more cheaply than the conventional materials obtainable according to the prior art.
The sealing of the gas spaces of a fuel cell from the ambient air and the other reactive gas in this case is essential to the safety and to the wide introduction of fuel cell technology. The use of sealing materials and their integration into the construction concept of the MEA is therefore of great importance.
Such construction concepts for membrane-electrode assemblies are described, for example, in U.S. Pat. No. 3,134,697 and in EP 700,108 A2. In these concepts, the membrane forms a rim extending over the electrodes and when the cell is sealed this is clamped between the cell plates and, if necessary, between further seals. However, membrane-electrode assemblies (MEAs) having such an overhanging membrane rim are susceptible to mechanical damage to the membrane during manufacture and assembly. Damage to the membrane easily leads to failure of the cell, since the membrane has to separate the gas spaces of the reactive gases hydrogen and oxygen from one another. Furthermore, the production of such products uses an increased amount of ion-conducting membrane material due to the area of the overhanging rim. Depending on the structure and construction of the MEA, up to 50% more membrane material (based on the active area of the membrane) is required. Thus, for example, an MEA having an active area of 50 cm2 (i.e. with dimensions of 7.1×7.1 cm) and a circumferential overhanging rim of 0.9 cm has a total area of 64 cm2. This corresponds to an additional area of 28% (based on the active area of 50 cm2) of membrane material being required. Ionomer membranes are organic polymers having a complicated structure and are therefore expensive. Larger membrane rims increase the material losses and thus finally make the total MEA product more expensive.
EP 586,461 B1 discloses a membrane-electrode assembly containing integrated sealing materials. This MEA has a five-layer structure and is composed of an anode consisting of a catalyst-coated gas diffusion layer (GDL), an ion-conducting membrane and a cathode which once again consists of a catalyst-coated gas diffusion layer. In contrast to the present patent application, no catalyst-coated membranes (“CCMs”) are used for producing this MEA. The MEA production process is significantly more inflexible and differs substantially from the present patent application. In the preferred embodiment of EP 586,461 B1, two layers of sealing material are required and in a further embodiment one layer of sealing material is employed, but considerable amounts of this are used since the sealing material is applied to the outside of the uppermost electrode (i.e. on the uppermost catalyst-coated gas diffusion layer) in order to produce the composite of the total MEA. Since anode, membrane and cathode each have to have a contact area for the sealing material, there is a large overlap zone which leads to a considerable loss of active MEA area.
EP 1,037,295 B1 describes the continuous production of catalyst-coated membranes by means of screen printing. The catalyst layers are printed selectively (i.e. in a particular pattern) on the membrane in the form of a tape. A margin which consists of membrane area but does not serve as active area is produced. The CCMs produced by this process are therefore expensive and incur relatively high materials costs.
It is therefore an object of the present invention to provide a catalyst-coated ion-conducting membrane which contains an integrated sealing material and can be produced inexpensively and simply. To allow inexpensive production, the ion-conducting membrane should be coated with catalyst over the entire front and/or reverse side and have no additional membrane rim. The catalyst-coated ion-conducting membrane should be able to be processed further in a simple process to produce a five-layer membrane-electrode assembly.